ISTANBUL — Airstrikes on rebel-held areas in the Syrian city of Aleppo destroyed a hospital supported by Doctors Without Borders, the aid group said Thursday, killing at least 14 patients and staff in the latest attacks that have all but unraveled a cease-fire accord.

A separate airstrike killed as many as 20 people, according to the Britain-based Syrian Observatory for Human Rights, amid mounting worries among international relief coordinators and diplomats that Syria’s conflict could be moving onto dangerous new ground with Aleppo as a pivot point.

The chief U.N. envoy to Syria broke it down in a stark tally covering recent days: An average of one Syrian killed every 25 minutes; one wounded every 13 minutes.

It was not immediately clear who carried out the air attacks that left more than 60 people dead since late Wednesday.

But the Syrian air force — backed by ally Russia — has stepped up its raids in Aleppo in recent days, striking both civilians and rebel factions despite U.N.-led efforts to restart peace talks in Geneva.

A rebel defeat in strategic Aleppo would strengthen the government’s position and pave the way for regime gains elsewhere. It also would mark a huge setback for rebel groups, including some backed by the West and